A plant disease caused by fungi of the family Peronosporaceae in the order *Peronosporales
. The pathogen penetrates the tissues more deeply than *powdery mildews
and produces a white or grey mealy growth on the leaves and stems of the host in humid conditions. This surface growth is composed of sporangiophores, which grow out from the stomata. Examples include downy mildew of vine (Plasmopara viticola), downy mildew of onion (Pero-nospora destructor), and downy mildew of lettuce (Bremia lactucae).